I have a 55 gallon tank that had 3 mystery snails until recently. They were all between 1-1.5 inches, and I have the tank pretty well sealed up where there aren't any gaps larger than about a quarter inch. One of my mysteries died a couple weeks ago, but the other two were looking healthy. Anyway, I went on vacation recently and left my wife at home to take care of the aquarium. She fed them all daily and swears she never left the top open. However, for the last 5 days, I haven't been able to find any more than 1 mystery snail. The one missing was actually my largest. I've looked everywhere around the tank, just in case it had escaped, but I haven't found, or smelled, any evidence of it. I guess it's possible that it crawled up into one of my aquascaping fake rocks and got stuck, but that would be odd , since they've lived over 3 months without doing that prior to this. I guess I have two other culprits. We have a 55 lbs dog would could have eaten it had it crawled out and died. We also had a cleaning service come to the house when I wasn't home, but I have a hard time imagining them finding a 1.5 inch snail on our floor without realizing that it came from the aquarium or at least telling us about it (they once notified me of dead stink bugs on my window sill). As of now though, I'm completely stumped. I've had fish completely disappear before, but I just assumed they were eaten by the rest of the fish. I never thought I'd lose complete track of a snail though.